Remove redundant prefix from UnbindReason

This is the last step to split InputMethodClient into multiple
classes.

Now that all the integer constants for UnbindReason are defined inside
UnbindReason, "UNBIND_REASON_" prefix is just redundant.

This is a mechanical refactoring. There should be no user-visible
behavior change.

Fix: 118040692
Test: prebuilts/checkstyle/checkstyle.py -f \
      frameworks/base/core/java/com/android/internal/inputmethod/UnbindReason.java
Change-Id: Iafce68b614dd85392d06af8726525a18b014dba0